Beaches buffer our shores from Mother Nature’s might

Most Kiwis
appreciate the recreational opportunities our beaches offer
– whether for beachcombing, surfing, walking the dog,
bird-watching, swimming or simply snoozing in the sun. But
did you know that beaches are also Nature’s way of
buffering and protecting seafront real estate and
infrastructure, such as parks and roads, against high winds
and waves during powerful storms or rough
seas?

If you live at the beach or visit it
regularly, you will have seen how sand on the beach goes
through cycles of erosion and accretion (build up).

The
erosion process can be dramatic – taking place when big
steep seas, combined with high tides, cause waves to pound
the shoreline. Big surges of water rush up the beach with
considerable force, scarping the dune face.

However, the
beach fights back by allowing a proportion of the uprush to
percolate into the sediments. As a result, the backrush has
less power than the uprush, protecting the beach from
erosion.

Gravel beaches are particularly good at
protecting themselves from erosion, says NIWA marine
geologist and coastal oceanographer Dr Terry Hume,
“because they are very permeable”.

“With a lot of
the uprush water lost into the gravel, the backrush has
little power to erode the beach face. Furthermore, the
powerful uprush carries with it gravel that gets tossed up
by the waves to build up a gravel ridge above high-tide
level. This ridge prevents waves tipping over the ridge,
creating further erosion.”

Sand stripped from the beach
and dunes during storms is carried out to sea by the
backwash and undertow to the shallow nearshore, where it is
deposited in banks. Surfers know this well as waves peeling
over these banks in shallow water provide excellent
surfing.

Dr Hume, a keen surfer himself, says the
formation of these banks is the beach fighting
back.

“When waves break on the nearshore banks they
absorb wave energy so that the waves have less power when
they arrive at the beach, buffering it from further
erosion.”

Accretion of sand takes place during calmer
periods of long, low sea swell and is very gradual. The
banks slowly migrate to the shore where they weld onto the
beach.

Sand that dries out at low tide is picked up by the
wind and blown into the dunes where is it is stabilised by
dune-binding plants such as spinifex and pingao. The
build-up of this buffer of sand in the beach and dunes
completes the erosion/accretion cycle. However, the
intensity of the next storm will determine whether the slow
fight back was enough to buffer against the next bout of
erosion.

“Understanding how beaches buffer themselves
against storms has taught us how to fight like Nature,”
says Dr Hume.

“Coastal management authorities are now
using the natural buffering processes of beaches in their
own approach to beach erosion control and preservation. For
example, beach nourishment (the process where sand lost by
erosion is replaced by sand from another source by
mechanical means) is often now used instead of hard
structures such as sea walls, which cause waves to scour the
beach in front. Dune conservation, by fencing and planting,
is also encouraged as a means to build up the buffer of
sand.”

Dr Terry Hume is a marine geologist
and coastal oceanographer at NIWA’s Hamilton office. His
research interests focus on large-scale coastal processes,
including the natural transport and storage of sand, coastal
hazards, and beach erosion; and developing and applying
beach and estuary classification for research and management
purposes. He holds an Honorary Associate Professorship at
the University of Auckland and an Honorary Lectureship at
the University of Waikato. He’s also a keen surfer.

Fast facts about
sand:

Origins: Rock or
shell

Time to form: Decades (soft shell,
such as cockles and horse mussels) to millennia (hard rock,
such as feldspar and quartz).

Distance
travelled: Hundreds of kilometres over millennia.
Longest journey in New Zealand is about 400km (Mt Taranaki
to North Cape).

• To qualify as sand, grains must
be between 0.06mm and 2mm in diameter.• 1200oC is the
temperature needed to convert quartz sand to
glass.• Quicksand is sand that has been so saturated
with water that the friction between sand particles is
reduced and they can no longer support weight.• Sand
can produce high- or low-frequency sounds under certain
conditions – nobody is really sure how. This strange
phenomenon occurs at some New Zealand beaches – such as
Waihi Beach (Bay of Plenty) when conditions are
right.

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